Two-component, aqueous polyurethane coating compositions are known and have been described in U.S. Pat. No. 5,075,370 and in copending applications, U.S. Ser. Nos. 7/644,180, 07/820,172 and 07/842,866. These coating compositions contain polyisocyanates, which may either be hydrophobic or hydrophilic, in combination with water dispersible or water soluble polyols. It has surprisingly been found that even though water is present in a substantial molar excess in comparison to the polyol, that it is possible to obtain coatings which have an excellent surface appearance, i.e., the coating does not contain bubbles from the generation of carbon dioxide from the reaction between isocyanate groups and water.
One of the disadvantages of these systems is that after preparation of the coating composition and prior to its application to a substrate, the isocyanate groups do react with water. This results in the formation of carbon dioxide which initially is present in the system in dissolved form, i.e., as carbonic acid. However, continued generation of carbon dioxide exceeds the amount which can present in dissolved form. This causes foaming in the coating composition and makes it difficult to obtain high quality coatings.
Accordingly, it is an object of the present invention to provide aqueous polyurethane dispersions which have an improved pot life and which may be cured at ambient temperature to provide coatings which possess the properties obtainable from prior art two-component, aqueous coating compositions.
This object may be achieved in accordance with the present invention as set forth hereinafter by the use of two-component coating compositions wherein one component is a water dispersible or water soluble polyol and the other an unblocked polyisocyanate which is neither water soluble nor water dispersible and which contains both isocyanurate groups and allophanate groups.
The use of polyisocyanates which are not water soluble or water dispersible has been described in U.S. Pat. No. 5,075,370 and in copending application, U.S. Ser. No. 07/820,172. However, neither of these references recognize that it would be possible to increase the potlife of the coating composition by using polyisocyanates which contain isocyanurate groups and allophanate groups.